Universal Mobile Telecommunication System UMTS is being developed to offer wireless wideband multimedia service using Internet protocol. The UMTS as a third-generation 3G mobile communication system combines among others streaming with a range of unique services to provide high-quality Internet content to the mobile users. Images, voice, audio and video content are example of mobile multimedia services, which are delivered to the users via media streaming techniques. It means once the content has been put onto a media server, it can be delivered for example via streaming to the end users.
The streaming service in a wireless network is provided both to a single user by means of the so-called unicast connections and to a group of users by means of the so-called point-to-multipoint or even multipoint-to-multipoint communication. The point-to-multipoint services pose high demands on the network infrastructure and may consume considerable amounts of bandwidth. Some examples of such services are video-conferencing, whiteboarding, real-time multi-user games, multimedia messaging, virtual worlds. This kind of point-to-multipoint applications use broadcast or multicast mode for transmission. Broadcast has the possibility of addressing a packet to all destinations like to every user on the network. Multicasting supports transmission to a group of the users in the network. Each user can register to a multicast group. When a packet is sent to a certain group, it is delivered to all users registered to that group. Multicasting is a service that permits sources to send a single copy of the same data to an address that causes the data to be delivered to multiple recipients. Under multicasting and broadcasting only one copy of a message passes over any link in a network and copies of the message are made only where paths diverge. In the following description the multicast is taken as an example. The application is applicable also to the broadcast applications.
A server in the IP network provides the multicast service. The server stores the content, which is to be sent to the user by means of a multicast transmission. In the UMTS the Multimedia Broadcast/Multicast Service, MBMS, is provided. The task of the server is fulfilled by means of the Broadcast Multicast-Service Centre MB-SC, which is an MBMS data source. The MBMS data may be scheduled in the MB-SC, for example for transmission to the users every hour. It offers interfaces to a content provider, so that said content provider can request data delivery to the users. The BM-SC may authorise and charge content providers. Detailed description of the BM-SC is given in 3GPP TR 23.846 “Multimedia Broadcast/Multicast Service; Architecture and Functional Description”.
In the wireless networks the end users are characterized by a variety of mobile terminals with a wide range of capabilities like for example display size. In current multicast solutions the source of data, like the server adapts the transmission conditions to the slowest receiver. This method is not sufficient when clients are charged for the service and for the bearer that is used for the service. In addition different radio-access networks make multiple maximum-access link speeds available. Because of the physical characteristics of the cellular radio networks, the quality and, thus, the data rate of ongoing connection will vary contributing to the heterogeneity problem.
Receiver-Driven Layered Multicast, S. McCanne, V. Jacobson and M. Vetterli, “Receiver-driven layered multicast”, In Proc. of ACM SIGCOMM'96, pages 117-130, Stanford, Calif., August 1996, is a well-known technology in the Internet and it solves some of the above-mentioned problems. Several improvements have been proposed. The basic idea is as soon as more capacity becomes available, the receiver changes a subscription level by joining to an additional multicast group and adapts the stream to the actual network conditions. The server provides a number of quality levels, and the member decides which quality level fits to his current network condition. In other words, if there is an unsatisfied client with the offered quality, a change to another group with the requested quality is performed. The user gets separately information only about the additional group.
Actually, receiver-driven sub-grouping can deal with changing network conditions, keeping the subjective quality at a reasonable level.
The clients need total knowledge on available multicast groups in order to make correct decision on which group to join. Moreover, offering different content in various groups need to be announced in the usual way using for example a Session Application Protocol SAP. In case of streaming applications, several coded variants of the same video clip are stored on a content server and send out to different multicast groups. Clients can then choose the variant that they prefer or that their terminals are able to receive and to process. The disadvantage is that with the usual way of announcement separated information about a multicast group is distributed. There are no possibilities to distribute information about dependences between the simple multicast groups. The users do not have any possibilities to change to another kind of content without performing an end-to-end protocol interaction. Therefore the problem is basically that there is no dynamic or flexible mechanism within the network to let clients zap fast between channels with different content. Further problem of the current solutions is that the user cannot easy hop between different groups with different quality levels.